AB1357 THE BEST CUT-OFF POINT FOR MEDIAN NERVE CROSS SECTIONAL AREA AT THE LEVEL OF PISIFORM BONE
Authors: A. Carones Esteves, P. Freitas, L. Brites, T. Santiago, M. Santiago, J. Rovisco, J. A. P. Da Silva
Journal: Annals of the Rheumatic Diseases (2022)

Background Carpal tunnel syndrome (CTS) is a focal neuropathy caused by compression of the median nerve (MN) at the wrist. Electromyography (EMG) is the gold standard for the diagnosis of CTS. Currently, the ultrasound (US) is frequently used as an initial screening exam by measuring the cross-sectional area (CSA) of the MN. The cut-off point of the CSA at the pisiform bone level to define CTS remains controversial with previous studies reporting values between 6.5mm 2 and 15mm 2 (1). Objectives The aim of this study is to determine the best cut-off point of the CSA for the diagnosis of CTS. Methods Cross-sectional study at a Tertiary Rheumatology Department including patients aged ≥ 18 years with symptoms compatible with CTS. Sociodemographic and clinical data, visual analogue scale for pain (VAS), Boston Questionnaire (BQ), and the results of EMG and US performed in each patient were collected. The EMG was performed according to the standardized protocol (sensory conduction velocity, sensory amplitude, distal sensory and motor latency), and the patients were categorized in 4 groups: normal, mild, moderate, and severe. A rheumatologist with expertise in imaging performed all the US evaluations by means of a 6–18-MHz (Siemens ACUSON S 2000) linear array transducer. The largest CSA of the MN was measured at the level of the pisiform bone. Receiver operating characteristic (ROC) curve was used to determine optimal cut-off values of the CSA taking the EMG result as the gold-standard. One-way ANOVA test was used to compare CSA between the 4 EMG groups. Results Fifty patients were included, 90% were female, mean age was 52.1 ± 10.8 years and median duration of symptoms was 28.0 (IQR 23.0-31.0) months. The mean VAS was 4.2 ± 2.9. In the BQ there was a mean symptom severity score of 2.4 ± 0.6 and a mean functional status score of 2.0 ± 0.9. One-way ANOVA showed that mean CSA values were significantly different in the 4 groups of patients. The Ryan-Einot-Gabriel-Welsch post hoc analysis showed that only the mean CSA of patients with severe STC is different from the remaining groups defined by EMG results. The best cut-off point for CSA at the pisiforme bone level for CTS diagnosis was 6.6 mm 2 with a sensitivity and specificity of 92.9% and 75.0% (AUC=0.9, P &lt;0.001). The positive and negative predictive values were 95.1% and 66.7%, respectively. For severe CTS diagnosis the best cut-off point for CSA was 12.3mm 2 with a sensitivity of 82.4% and a specificity of 72.7% (AUC=0.8, P &lt;0.001). The positive and negative predictive values were 60.9% and 88.9%, respectively. Conclusion In our study we found that the best cut-off point of the CSA was 6.6mm 2 for distinguishing patients with/without CTS based on EMG alterations, with a high sensitivity and moderate specificity. This is a lower cut-off value than usually used in clinical practice and could be explained by small sample and the greater number of patients with mild and moderate STC on EMG. References [1]McDonagh C, Alexander M, Kane D. The role of ultrasound in the diagnosis and management of carpal tunnel syndrome: A new paradigm. Rheumatology (Oxford, England). 2014;54. Disclosure of Interests None declared